[stage] Warn if perspective's z_far - z_near is 0
[stage] Warn if perspective's z_far - z_near is 0

Since we have to do (z_far - z_near) and use it in a division we
should check that the user is not passing a value that would
cause a division by zero.

Distrotech/clutter,spatulasnout/clutter,djdeath/clutter-android,heysion/clutter-clone,spatulasnout/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,ebassi/clutter,djdeath/clutter-android,Distrotech/clutter,ebassi/clutter,djdeath/clutter,djdeath/clutter-android,djdeath/clutter,GNOME/clutter,dlespiau/clutter,rkudiyarov/clutter_osx,tardyp/clutter,kerrickstaley/clutter-vala,djdeath/clutter,rkudiyarov/clutter_osx,collects/clutter,djdeath/clutter-multithreaded,collects/clutter,GNOME/clutter,tardyp/clutter,collects/clutter,jigpu/clutter,GNOME/clutter,GNOME/clutter,spatulasnout/clutter,rkudiyarov/clutter_osx,heysion/clutter-clone,spatulasnout/clutter,collects/clutter,Distrotech/clutter,djdeath/clutter,nobled/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,jigpu/clutter,dlespiau/clutter,spatulasnout/clutter,kerrickstaley/clutter-vala,djdeath/clutter,djdeath/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,ebassi/clutter,rkudiyarov/clutter_osx,jigpu/clutter,djdeath/clutter-android,GNOME/clutter,djdeath/clutter-multithreaded,jigpu/clutter,tardyp/clutter,jigpu/clutter,nobled/clutter,collects/clutter,jigpu/clutter,ebassi/clutter,heysion/clutter-clone,ebassi/clutter,djdeath/clutter-android,heysion/clutter-clone,dlespiau/clutter,nobled/clutter,Distrotech/clutter,Distrotech/clutter